Digital subscriber line (DSL) technologies, such as HDSL and HDSL2, are currently used by telecommunication service providers to deliver services such as ADSL, SHDSL, ISDN and POTS to terminal access platforms at remote (customer premises) installations. In addition to sending voice and/or data over the telephone lines, the service provider may use the same wireline pair to transport main or auxiliary power to the remote terminal equipment from the telephone company's central office (CO). This method of remote wireline powering, commonly referred to as ‘loop’ or ‘span’ powering, offers significant flexibility in deployment and reliability of service, as it permits the phone companies to power terminal equipment located up to several miles away from the central office.
POTS service requires the availability of a (relatively large) ringing voltage at the remote access platform which, in the case of a line powered device, must be supplied over the same wireline pair. Unfortunately, the power delivered by the ring generator (and thus delivered to it) can be expected to fluctuate substantially over time, because of the time-varying nature of the ringing waveform and the statistically changing load requirements. The peak input power required by a remote access platform can exceed the power available to it, due to the peak load demand on the ring generator; still, the ring generator must be capable of meeting reasonable load demands for the system to be viable.
The amount of power available to the remote access platform and the ringing load is limited in accordance with the expression:PRT=(VCO)2/4RLwhere PRT is the power delivered to the remote access platform (RT), VCO is the central office span voltage, and RL is the line resistance. VCO is limited by telecom industry standards, safety requirements, and voltage breakdown limitations of some twisted pair telephone cable. Therefore, arbitrary increases in span voltage to meet powering requirements of the remote equipment are not feasible.
A second problem is that noise associated with the generation of the ringing voltage can mix with the DSL signal, and cause significant performance problems for the DSL signal. Some DSL systems are very sensitive to ringing voltage ripple because the 20 Hz ring frequency is high enough to interfere with the signal, yet too low for the analog or digital filters of the signal path to attenuate. Passive power filtering between the remote terminal's input and the ringing generator requires a physically large circuit and may not be practical, because of the low frequency and the amount of attenuation required. For a 50 Vrms ringing voltage, 80-100 dB of attenuation may be required at 20 Hz.
Advantageously, these problems are successfully addressed for single subscriber line applications by the invention disclosed in co-pending U.S. patent application Ser. No. 09/442,299 (hereinafter referred to as the '299 application), now U.S. Pat. No. 6,690,792 B1, entitled: “Active Decoupling and Power Management Circuit for Line-Powered Ringing Generator,” by S. Robinson et al, assigned to the assignee of the present application, and the disclosure of which is incorporated herein. In particular, the '299 application discloses a ringing voltage power management circuit that generates an isolated high-value voltage derived from the powered wireline through a linear current-limiting circuit, and uses this isolated high-valued voltage to charge a storage capacitor, that serves as an energy reservoir for the ringing voltage generator for the subscriber line. The storage capacitor and the current-limiting circuit isolate the peak power drawn by the ringing load from the telephone line's remote terminal input and translate the 20 Hz ripple to the sub-Hz oscillations of the ringing cadence (ring on/off cycle).
While the linear nature of this current limiting circuit results in a power dissipation that is acceptable for single subscriber line applications, it is not feasible for use with a remote access platform serving considerably more than one line (e.g., twenty-four lines in one present day remote terminal device) and having a potentially much higher ringing power, where a linear (dissipative) circuit to limit peak power is not acceptable.
This power demand problem also occurs in hybrid schemes that are capable of extracting both local power and span power for operating the remote terminal equipment. For example, co-pending U.S. patent application Ser. No. 09/378,382, (hereinafter referred to as the '382 application), now U.S. Pat. No. 6,584,197 B1, entitled: “Power-Limited Remote Termination Converter with Wetting Current and Emergency Power Operation for Digital Data Services Equipment,” by R. Boudreaux Jr., assigned to the assignee of the present application, and the disclosure of which is incorporated herein, describes a dual power management system that monitors the power availability for and power requirements of the remote termination equipment.
For normal operation, local power is used to meet all terminal needs, except for a small amount of power drawn from the span for wetting current purposes. During emergency operation (loss of local power) the functionality of the remote unit is reduced to conserve power, so that only lifeline POTS is available. In this mode, power is derived from the phone line up to a maximum value that prevents the line from collapsing, and satisfies current industry proposals. In addition, an auxiliary capacitor stores a limited quantity of energy from the local utility interface or the span, to enable extended power operation (including ringing the phone).
Now although the power management scheme described in the '382 is a relatively cost-effective architecture, due to the low power and simultaneous dual power input nature of the remote terminal, it is not particularly suited for higher power and single input (wireline only) applications.